1. Field of the Invention
The present invention relates to a fluorine-containing elastomer composition, and more particularly to a cross-linkable elastomer composition of fluorine-containing elastomer having cyano groups as cross-linkable groups.
2. Related Prior Art
Japanese Patent Publication (Kokai) 59-109546 discloses a fluorine-containing elastomer composition, which comprises a terpolymer of tetrafluoroethylene, perfluoro(methyl vinyl ether) and cyano(perfluorovinyl ether) represented by the following general formula:
CF2xe2x95x90CF [OCF2CF(CF3)]nO(CF2)mCN
wherein n is 1 to 2 and m is 1 to 4, and a bisaminophenyl compound represented by the following general formula as a curing agent: 
wherein A is an alkylidene group having 1 to 6 carbon atoms, a perfluoroalkylidene group having 1 to 10 carbon atoms, a SO2 group, an O group, a CO group, or a carbon-carbon bond capable of directly coupling two benzene rings; and X and Y each are a hydroxyl group or an amino group.
However, the cyano(perfluorovinyl ether) to be copolymerized as a cross-linking site monomer in the terpolymer has been synthesized through many steps, as shown in U.S. Pat. No. 4,138,426 and thus is not an industrially preferable raw material and also fails to give a satisfactory compression set at an elevated temperature.
U.S. Pat. No. 3,546,186 discloses a terpolymer obtained by copolymerization of tetrafluoroethylene and perfluoro-(methyl vinyl ether) with perfluoro(xcfx89-cyanoalkyl vinyl ether) represented by the following general formula as a cross-linking site monomer:
CF2xe2x95x90CFO(CF2)nCN
wherein n is 2 to 12, and also discloses that the cross-linking is carried out by tetraphenyl tin.
The perfluoro(xcfx89-cyanoalkyl vinyl ether) used as a comonomer component in the terpolymer can be readily synthesized from a corresponding carboxylic acid ester with a high selectivity, and thus is an industrially utilizable compound, but the cross-linking of the resulting terpolymer is carried out through cyclization and terpolymerization of the cyano groups by tetraphenyl tin and thus not only press vulcanization requires such a longer time as 18 hours at 160xc2x0 C., but also its industrial utilization still has problems in the cross-linking such as use of highly toxic tetraphenyl tin as a cross-linking agent and use of expensive silver oxide as a promoter. Furthermore, it fails to give a satisfactory compression set at an elevated temperature.
An object of the present invention is to provide a fluorine-containing elastomer composition comprising a copolymer containing a cross-linking site monomer capable of being readily synthesized with a good selectivity, as copolymerized, the composition being capable of giving a rubbery vulcanization product with a good processability and good physical properties by a commercially available cross-linking agent free from any safety problem.
According to the present invention, there is provided a fluorine-containing elastomer composition, which comprises a terpolymer of tetrafluoroethylene, perfluoro(lower alkyl vinyl ether) and perfluoro(xcfx89-cyanoalkyl vinyl ether) represented by the following general formula:
CF2xe2x95x90CFO(CF2)nCN
wherein n is an integer of 2 to 12, and a bisaminophenyl compound represented by the following general formula as a curing agent: 
wherein A is an alkylidene group having 1 to 6 carbon atoms, a perfluoroalkylidene group having 1 to 10 carbon atoms, a SO2 group, an O group, a CO group, or a carbon-carbon bond capable of directly coupling two benzene rings; and X and Y each are a hydroxyl group and an amino group.
The terpolymer of tetrafluoroethylene, perfluoro(lower alkyl vinyl ether) and perfluoro(xcfx89-cyanoalkyl vinyl ether) for use in the present invention is a terpolymer obtained by copolymerizing a copolymer containing about 30 to about 75% by mole of tetrafluoroethylene and about 70 to about 30% by mole of perfluoro(lower alkyl vinyl ether) with about 0.1 to about 5% by mole, preferably about 1 to about 2% by mole of perfluoro(xcfx89-cyanoalkyl vinyl ether) as a cross-linking site monomer. The terpolymer can contain various vinyl compounds or olefin fluorides, as copolymerized to such an extent as not to inhibit the copolymerization reaction and not to deteriorate physical properties of vulcanization product.
Copolymerization reaction can be carried out by emulsion polymerization or suspension polymerization in an aqueous medium or by solution polymerization in a perfluoro compound as a solvent, using a free radical generator. Emulsion polymerization is preferable for obtaining a terpolymer of larger molecular weight.
In the emulsion polymerization in an aqueous medium, ammonium persulfate, potassium persulfate, etc. are used as a free radical generator. These free radical generators can be also used as a redox system together with a reducing agent such as ammonium salts, sodium salts, potassium salts, etc. of sulfurous acid, thiosulfuric acid, etc. Ammonium salts of perfluorated carboxylic acids can be usually used as an emulsifying agent, and ammonium perfulorooctanoate is particularly preferable. A pH-adjusting agent such as Na2HPO4, NaH2PO4, K2HPO4, KH2PO4, etc. can be usually used.
The copolymerization reaction can be carried out according to one charge polymerization procedure where predetermined amounts of the respective monomers are all charged into a reactor at one time and subjected to the polymerization reaction or according to a polymerization procedure of continuous addition at constant rates, where the respective monomers are continuously added to a reactor at constant rates under a constant pressure. The latter polymerization reaction procedure is preferable for obtaining polymers with good physical properties, where the polymerization pressure is kept under about 2 to about 50 kg/cm2 gage and the polymerization temperature at about 40 to about 80xc2x0 C. during the continuous addition at constant rates.
After the polymerization reaction, the resulting aqueous latex is salted out by an aqueous solution of water-soluble electrolyte such as sodium chloride, magnesium chloride, etc., recovered by filtration and dried, whereby a terpolymer can be obtained. In place of salting-out, the aqueous latex can be freezed, recovered by filtration and dried.
About 0.5 to about 5 parts by weight, preferably about 1 to about 2 parts by weight, of the bisaminophenyl cross-linking agent represented by the above-mentioned general formula, for example, as given below, can be added to 100 parts by weight of the terpolymer: 
The fluorine-containing elastomer composition containing the above-mentioned components as essential ones can be further admixed properly, when required, with an inorganic filler such as carbon black, silica, etc., an acid receptor such as oxides, hydroxides or stearates of divalent metal, litharge, etc.: and necessary additives. The composition can be prepared by mixing in an open roll, etc. The cross-linking can be carried out by heating to about 160 to about 250xc2x0 C. for about 10 to about 60 minutes. Secondary vulcanization, if required, can be carried out preferably in an inert gas atmosphere, for example, in a nitrogen gas atomosphere.
According to the present invention, there is provided a fluorine-containing elastomer composition comprising a copolymer containing a cross-linking site monomer capable of being readily synthesized with a good selectivity, as copolymerized, the composition being capable of giving a rubber vulcanization product with a good processability and good physical properties by a commercially available cross-linking agent free from any safety problem.